1. Field of the Invention
The present invention relates to a leak detecting circuit for detecting a leak between the body of a vehicle including a device supplied with a direct-current voltage by a direct-current high-voltage power source and the direct-current high-voltage power source.
2. Description of the Related Art
A direct-current high-voltage power source (hereinafter a high-voltage battery) included in an electrically driven vehicle such as an electric car, a hybrid vehicle, or a fuel cell vehicle is usually not connected to a ground because when the negative electrode, the positive electrode or the like of the high-voltage battery is grounded, contact of a person engaged in service or the like with the high-voltage battery or a failure of the high-voltage battery may lead to an electric shock or a serious damage, and are thus dangerous. When a leak (ground fault) occurs between the high-voltage battery and the body of the vehicle as a result of external contamination such as mixing in of foreign matter, or water drop intrusion between the positive electrode or the negative electrode of the high-voltage battery and the body of the vehicle, because the leak is dangerous, the leak is detected, and an abnormality lamp is lit, for example, to warn a person engaged in service.
Related art relating to leak detection includes Patent Document 1. In Patent Document 1 (Japanese Patent Laid-Open No. 2007-256114), a protective resistance 10, an FET 20, a detecting resistance 30, and a variable direct-current power source 40 are connected in series with each other between a body ground and the negative electrode of a high-voltage battery, and a fixed voltage is applied to the gate of the FET 20 with the negative electrode of the high-voltage battery as a reference, to turn on the FET 20. In the case of a high potential side leak, in which a leak occurs between the positive electrode of the high-voltage battery and the body ground, a leak current flowing from the positive electrode of the high-voltage battery to a high potential side leak resistance to the body ground to the protective resistance 10 to the FET 20 to the detecting resistance 30 to the variable direct-current power source 40 to the negative electrode of the high-voltage battery is detected by a voltage across the detecting resistance 30, and the high potential side leak is detected.
At this time, because of insusceptibility to change in voltage of the high-voltage battery, the reference voltage for leak detection (reference voltage) of the variable direct-current power source 40 is changed to two voltages, for example 0V and 10-odd V, the leak current is calculated from voltages across the detecting resistance 30 at the two different reference voltages, the high potential side leak resistance is calculated from a current difference of the leak current or a voltage difference between the voltages across the detecting resistance 30, and the high potential side leak is determined.
However, Patent Document 1 has the following problems because a fixed gate voltage is applied to the FET 20, the voltage across the detecting resistance 30 is detected while the voltage (reference voltage) of the variable direct-current power source 40 is changed, and a leak, for example a high potential side leak is detected.
A minute leakage current (on the order of a few μA, for example) proportional to a voltage between the gate and the source of the FET 20 due to a resistance between the gate and the source of the FET 20 flows from the gate to the source of the FET 20. This leakage current flows into the detecting resistance 30 from the source of the FET 20. Therefore, the leakage current flows through the detecting resistance 30 in addition to the leak current. The leak current is limited by the protective resistance 10 to become a minute current (on the order of a few μA, for example) for protection from a hazard.
Because a fixed gate voltage is applied to the gate, when the reference voltage of the variable direct-current power source 40 is changed (changed from 0 V to 10-odd V, for example), the source voltage is changed by 10-odd V with the change in reference voltage of the variable direct-current power source 40. However, because the gate voltage is fixed, the voltage between the gate and the source is changed by 10-odd V, so that the leakage current proportional to the voltage between the gate and the source varies greatly.
The voltages across the detecting resistance 30 which voltages are detected under different reference voltages of the variable direct-current power source 40 each include a different error due to a different leakage current. Therefore, the error of the leakage current cannot be cancelled by a current difference or a voltage difference, and a detection error due to the difference in leakage current is included. Because of the minute leak detection current, the detection error due to the difference in leakage current cannot be ignored, and leak detection accuracy is degraded.